sourceOver function
Composites src over dst using Porter-Duff SourceOver.
When src is a translucent UvRgb, this blends it over dst using
integer arithmetic and returns an opaque UvRgb. Non-RGB colors fall back
to source replacement semantics.
Implementation
UvColor? sourceOver(UvColor? src, UvColor? dst) {
if (src == null) return dst;
return switch (src) {
UvRgb() => _sourceOverRgb(src, dst),
_ => src,
};
}